1. Field of the Invention
The present invention relates to a device for sealing a thermoplastic material by heat. The thermoplastic material generally includes a plastic film, which may be a single film, a laminated composite film or a sheet of metal such as aluminum coated by plastics. One of the lapped materials to be sealed is not necessarily thin.
2. Description of the Prior art
In general, heat sealing devices include a rotary type in which a heat sealing member heated by a heater is made in the form of a disc or a roller and a movable or a stationary type in which the heat sealing member is made in the form of a long block.
However, since the heat sealing member made of metal is directly heated by a heater alone in either of the aforementioned devices, the temperature on its sealing surface tends to vary in portion and in time and cannot be kept uniform depending on the quality and the form of the heat sealing member leading to uneven sealing of the film and to various defects caused by inferior sealing of the film. In such a prior art device, further, since thermal conductivity in transferring heat to the sealing surface is low, the device must be set at a high temperature previous to starting of sealing operation in consideration of temperature drop, and as a result, the sealing condition varies as time passes. Such a problem becomes especially conspicuous when a film having high heat absorption capacity or low-temperature melting characteristic is to be sealed.